Many different methods have been developed for forming single crystal epitaxial semiconductor films. Epitaxy is the regularly oriented growth of a crystalline substance on a crystalline surface. Single crystal films frequently have superior properties relative to other kinds of films, such as polycrystalline and amorphous films. Homoepitaxy is the growth of a crystalline film on a crystalline surface of the same substance. Heteroepitaxy is the growth of a crystalline film on a crystalline surface of a different substance. Chemical vapor deposition (CVD) processes, and to a lesser extent, physical vapor deposition processes, commonly are used to grow or deposit single crystal semiconductor layers on a crystalline substrate. The quality of the single crystal epitaxial films depends upon a number of different factors, including good lattice match between the film and the substrate, proper growth temperature, and proper reactant concentrations.
For many applications, polycrystalline or amorphous films are acceptable or even more desirable than epitaxial films. For example, many protective films are polycrystalline films, which may be characterized by high hardness, high corrosion resistance, and high oxidation resistance. Amorphous films (e.g., oxides, nitrides and glasses, such as silicon dioxide) also serve a number of useful functions, including electronic passivation, insulation and dielectric functions. Current device performance requirements, however, require that most or all of the active devices of an integrated circuit be formed in a single crystal semiconductor region. This requirement typically limits the integrated circuit devices to two-dimensional structures on a substrate surface.
Numerous attempts have been made to extend semiconductor device fabrication techniques to three-dimensional structures by growing single crystal films over amorphous films used as insulators in two-dimensional integrated circuits. For example, U.S. Pat. No. 4,686,758 describes a localized overgrowth process, in which seeding from a single crystal silicon substrate is used to grow single crystal silicon layers over an amorphous silicon dioxide gate layer. The localized overgrowth process involves etching a window in the silicon dioxide layer down to the single crystal silicon substrate, and growing an epitaxial silicon film upwardly from the substrate in the window. Localized overgrowth of single crystal silicon occurs when the selective epitaxial growth reaches the top surface of the silicon dioxide window. U.S. Pat. No. 6,103,019 describes a method of forming a single crystal film from a seed layer implanted in a non-crystalline surface by high-dose implanting of a nucleating species through a single crystal mask having appropriate channeling directions spaced at desired lattice constants. In zone melting recrystallization processes, a single crystal semiconductor layer may be formed on an amorphous layer by depositing a polycrystalline or amorphous semiconductor layer, melting the deposited layer with a laser or other energy source, and allowing the melted layer to re-crystallize, randomly or from a seed, by superposing a temperature gradient. Still other single crystal forming processes have been proposed.